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XML's architects at the W3C (World Wide Web Consortium) borrowed heavily from Standard Generalized Markup Language (SGML), a standard for defining tag sets for electronic text. But SGML was designed primarily for large-scale document publishing, not for the Web. As a result, several artifacts have crept into XML, spurring the need to modify some of its basic features. The DTD (document type definition) is a perfect example. It's adequate for defining elements in a publishing environment, but XML is moving into mainstream programming applications. As a result, developers need to associate element data with richer data types than those DTDs offer.

In addition to specifications that extend XML, the W3C Web site (www.w3.org) is working on enough XML-based specifications to make your head spin, including everything from scalable vector graphics to synchronized multimedia.

There's even a reformulation of HTML called XHTML, which makes Web page markup XML-compliant. A good summary of these efforts is available at XML.com, a site sponsored by O'Reilly & Associates.